The present application is based on DE 10 2006 040 459 A 1. It initially details the prior art in commonly available slewing gear controls.
It subsequently refers to the slewing gear as a device, where a crane superstructure with a corresponding cantilever can freely rotate atop a stationary support structure.
Power is normally provided through a hydraulic motor which positions the superstructure to the support structure through a gearbox with appropriate gearing.
When in operation, the rotational movement must be controllable at a very slow speed while at the same time capable of achieving high speeds to accommodate appropriate work cycles. The dynamic characteristics of the slewing systems vary strongly, dependent on cantilever lengths, reach and load weights. The requirement for the operation of the crane is also determined by its use on construction sites.
It requires a high degree of maneuverability due to resolution and conversion.
The slewing gear controls usually found in moveable cranes are designed as “closed and open circuits.” Volume flow- or pressure regulated systems are used within the “open circuits.”
In the “closed circuit” a variable displacement pump delivers fluid within a hydraulic circuit directly to the hydraulic motor without having an additional distribution point in the system. The returning oil is directly routed to the pump. Any leakage is fed into the respective return flow area by means of another auxiliary pump.
The delivery volume of the pump determines the speed of the rotational movement. The control valves within the pump regulate the volume, depending on the demand. The pump also determines the delivery direction and thus the rotational direction of the slewing gear. The mechanical/hydraulic valve system makes it possible to pivot the displacement unit of the pump from a maximum position through the neutral position to another maximum position and thus to change a continuous delivery stream from one outlet to another. At the same time the intake sides at the pump change also.
An advantage of this control principle is the fixing of the slewing gear, which to some degree prevents a turning when lateral forces are applied since the hydraulic motor rests on the pump and subsequently on the diesel motor. The fixing of the slewing gear from the “closed circuit” generates an immediate slowing of the rotational speed when the control is reduced. This requires the operator to pay close attention to the rotational movement.
Additional advantages include the positive energy balance as well as the delivery volume, which is determined by the pump geometry, and thus the possibility to move into precise positions.
The increased oil leakage from the hydraulic motor and pump affect the lateral forces negatively since it causes an unwanted drifting of the slewing gear. An additional dynamic rotary brake is required, despite the “closed circuit.”
In the “open circuit” a fixed displacement pump usually delivers oil from the tank over a proportional valve to the slewing gear motor. The returning oil flows to the tank through the proportional valve. The valve determines the rotational direction and the delivery volume to the slewing gear motor. Both are controlled by proportional signals depending on the demand.
The proportional valves can function as throttle valves or even as pressure balance valves, which subsequently assures a regulation of the delivered stream independent of the pressure.
Basic throttle controls are perfectly appropriate when the slewing gear is being operated in a very dynamic manner; however they are less controllable with changing loads.
Delivery stream regulators can control or regulate minute speeds independently of the load but are inappropriate for dynamic operations with counter steering. Due to the free-wheel mechanism of the sliders the load hook automatically self-centers exactly above the load when the brake is released and the load hook is raised.
A significant disadvantage of “open circuits” is in the deliberate stopping of a movement. Braking is impossible while using the proportional valve since variable loads require different braking forces for a specific braking distance.
Since this option does not exist, in their neutral position the control slides are always in the freewheel mode of the slewing gear motor. A dynamic brake is required for stopping. These brakes are mainly designed as mechanical multiple disk brakes, their handling being equally problematic when it becomes necessary to brake significantly varying loads.
DE 10 2006 040 459.9 A1 reveals a hydraulic control circuit for the control of a hydraulic system which controls a drive, in particular for controlling a hydraulic motor for driving a slewing gear of a crane superstructure, which is characterized in that in both operating lines between a hydraulic fixed displacement pump and a hydraulic motor for controlling the slewing gear, separately activatable directional pilot valves, as well as separately activatable directional control valves, are arranged which are controllable by means of the inflow- and outflow ratio from and to the hydraulic motor and thus control the rotational direction thereof.
In such a hydraulic control circuit two versions are also conceivable:                1. Directional control valves that are “open” in their non-activated mode, i.e. flow through the operating lines and return line to the tank        2. Directional control valves that are “closed” in their non-activated mode, i.e. no flow through the return line to the tank        
In the case of version 1, loss of energy necessary for activating the directional control valves in this case means an uncontrollable continuous rotational movement based on the inertia of a slewing platform and/or the starting of a rotational movement based on uneven load distribution of a slewing gear, which is at rest. Uncontrollable continuous rotational movements as well as the starting of an unintended rotational movement will pose a safety hazard.
In the case of version 2, loss of the energy necessary for activating the directional control valves in this case means the closure of the drive's discharge lines. This causes the abrupt deceleration of the rotational movement. This involves the risk of mechanical overload on the machine and/or tipping of the machine.
The object of the present invention is to develop a hydraulic circuit in such a manner that even in case of a partial or complete failure of the activation of slewing gear, lifting gear or retractable lifting gear, the machine operator will be able to decelerate and ultimately stop the slewing gear, lifting gear or retractable lifting gear.